Sound recording system



Oct. 28, 1941. B. KREUZER 2,260,717

SOUND RECORDiNG SYSTEM Filed March 24, 1939 5 Sheets-Sheet 1 mun! iunm mm! mm! nmlml llllmm I I I l I I I n l I l 2 l 3427? Zea/2E2, M/XM Oct. 28, 1941. KREUZER SOUND RECORDING SYSTEM Filed Mar ch 24, 1939 3 Sheets-Sheet 3 R1 IkACK a2 1007.11000LAT/0N INPUT VOLTAGE fiAerolv Ken/Z512,

lAMPz/F/Ez {Mahmu Patented Oct. 28, 1941 Barton Kreuzer, Los Angeles,'-Calli., assignor to Radio Corporation of America, a corporation of Delaware Application March 24, 1939, Serial No. 263,866

' 16 Claims. (01. 179-1003) This invention relates to sound recording systems and particularly to sound recording systems wherein noise reduction is applied during the recording.

Sound recording systems of the variable area and variable density types are well known in the art. In the area type of system the film is lightimpressed with a beam varying transversely of the sound track portion of the film, while in the density type, the film is light-impressed with a beam of constant length and width, but of varying intensity. The application of noise reduction to both types of systems is also well known wherein all or substantially all of the unmodulated track area is opaque in the final print, or the modulation of the variable intensity beam varies between a point on the density characteristic curve which provides the least distortion at the highest amplitude, and a point at one 'end of the characteristic curve.

The present invention is particularly directed to the optimum practical method of and apparatus for applying the noise reduction action;

An object of the invention, therefore, is to improve the noise reductionaction oi the noise reduction portion of a sound recording system.

Another object of the invention is to control 1 the noise reduction portion of the apparatus to reduce the light transmission through the sound Referring specifically to the sound recording type,

of system which produces a variable area duplex track, two small transparent traces are found in the final print at times of no signal, the outer portions being under control of noise reduction shutters and the central portion being under control of the movements of the galvanometer Lg mirror. In this type of system, the noise reduc tion shutters are actuated with direct current obtained from rectifying a portion-of the signal being impressed on the galvanometer. Thus, ac-

tuation of the shutters occurs substantially'simultaneously with [the actuation of the galvanometer. Therefore, and particularly when loud sounds are suddenly impressed on the galvanorneter, the shutters fail to clear the track area to accommodate the peaks of the modulations, and so-called clipping of the peaks occurs.

On the other hand, if the shutterswere actuated with sufficient rapidity to clear the suddenly applied high amplitude sounds,- the traceor traces made by the shutters would introduce audtrack print to a minimum during times of no signal and to prepare the sound track area in anticipation of the signal.

A further object of the inventionis to actuate the noise reduction portion of the system at different speeds depending upon the amplitude of the signal being recorded.-

A further object of the invention is to automatically actuate the noise reduction shutters at different rates according to the amplitude of the signal.

'A further object of the-invention is to vary the rate of the application of noise reduction in accordance with the amplitudeof the signal.

A'further object of the invention is to produce a sound record having the optimum application of noise reduction. a 3' Although the novel features which are believed tobe characteristic of this invention are pointed out with particularity in the claims appended herewith, the manner of its organization and the mode of its operation will be better understood byreferring to the following description readin conjunction with .the accompanying drawings forming a part thereof, in which Figure l is a diagrammatic arrangement of the sound recording system showing one embodiment of the invention; I

Figure 2 is a sound track print resulting from one mode of operation of the system of Fig. 1;

Figure 4 is a diagrammatic drawing showing another embodiment of the invention;

' of the system of Fig. '7.

Figure 5 is a sound track record resulting from the system of Fig. 4; v

Figure 6 is a graph illustrating the operation of the system of Fig. 4 I I Figure 7 is a diagrammatic drawing showing a third embodimentof the invention; I Figure 8 isa sound track record resulting from the system of Fig. 7; and

Figure 9 is a graph illustrating the operation "Referring now to .Fig. 1, tion of the drawing diagrammatically represents a variable area recording system of the standard duplex record type, such as that disclosedby the right-hand por- I1 having a slit I8 therein, from which it is collected by objective lenses I9 and impressed upon the sound track portion 20 of a film 2|. "The above-described apparatus is the 'usuahrecording system for producing standard variable area duplex tracks.

44, 45 and 46 may be located directly at the recorder, while the other two controls may be located at the amplifier-rectifier unit, whichever is more convenient from the operators standpoint.

Referring now to Fig. 2, the type of sound record made by the use of the circuit so far described is shown. This record is a print in which noise reduction has been applied to the negative during recording, the outer opaque areas resulting from the operation of the shutters I4, and the inner opaque area resulting from the vibration or. non-vibration of the light beam impressed Y on the mask I1.

The sound to be recorded is detected by, a microphone 24 and translated intoeIectrical cur rents which are amplified by an amplifier 25 i and impressed upon a coil of the galvanometer I2 over conductors 2i. The impression of these currents on the galvanometer causes the mirror II to follow the instantaneous values of the frequencies of the signal and vibrate the light beam normal to the slit it. A portion of the current output of the amplifier is impressed through transformer 32 upon a rectifier 28 having a timing filter circuit consisting of a series resistance 29, a shunt resistance 30 and a shunt capacitance 3!. This timing filter circuit is disclosed and claimed in my Patent No. 1,999,700, issued April 30, 1935. The rectified output of rectifier 28, after passing the filter, is impressed upon a direct current amplifier 33,'the output of which is impressed upon the actuating coil I5 of the noise reduction shutters i4. Bias for the amplifier 33 is provided from any suitable voltage bias source such as a battery 34 shunted by a potentiometer 35, the setting of the latter determining the initial opening or separation of the shuters I4 by controlling the amount of plate current from the tube 33 to the coil I5.

Bridged across the resistance 30 and capacitance 3| is a manually operated shunting circuit consisting of a variable resistance 31, a sourceof potential 38 and a manual key 39. Normally the switch 39 may be open, but when closed, the potential of the source 38 may oppose or buck the potential from battery 34 ,applied'to the grid of tube 33, causing the shutters tobe drawn closer together. The reverse switching action may be obtained by reversing the polarity of source 38. The same result of drawing the shutters closer together may also be accomplished by a switch contact 40 which operates between points 4| and 42. That is, the point 42 may be the normal operating point for the tube 33 to provide a certain spacing for the shutters I4, while movement of the contact 40 to the point M will change the bias ontube 33 and, consequently, provide a narrower spacing, the results of which will be disclosed hereinafter.

To also vary the spacing of the shutters I4in a similar manner, a circuit shown in dotted lines is bridged directly across the coil winding I5, this circuit consisting of a potential source such as a battery 44, a resistance and a manually operated switch 45. Thus by closing the switch 43, the potential from the battery 44is such as to decrease the spacing between the ends of the shutters I4. Any of the shutter spacing controls just described, produce the same result at the shuttersthat is, vary their separation. There may be some advantage, however, in the use of one particular control, for instance, the circuit At times of no signal the track i lflcars as shown above the points aa, the track area having two narrow transparent zero lines or traces, the negative film being advanced in the direction of the arrow during recording. The variation in the width of the transparent traces at points aa is caused by an operator actuating either of switches 46 or 46, depending'on which control circuit is being used. The operator watches the sound source and, anticipating the beginning of the signal, will open or close either of switches 39, 46, depending on the polarities of the respective batteries, or throw the switch 40-to the contact 42. In doing so, the noise reduction shutters I4 are allowed to separate and the transparent traces widened, as shown between the points a-a and b--b'.

This action, as may be noted, prepares the sound track for the application of the signal shown by the modulations c, which, as shown for purposes of illustration, increase in amplitude to a maximum value and then decrease to zero. Simultaneously, the noise reduction shutters I4, receiving rectified current from the rectifier 28 through the amplifier 33, continue to separate as the signal amplitude increases to provide the necessary track area for the modulations c. When the modulations decrease in amplitude, the noise reduction shutters, through the timing elements 29, 30 and 3 I, begin to close, but at a slower rate than when they were opened, so that the closing time is longer, as shown between the points d-d and ee. Now when the recording operator notes that the signal will be absent for some time, he actuates the appropriate switch and the shutters are drawn closer together as shown at ;f This action produces the maximum desirable opaque area during times of no signal and the maximum noise reduction while maintaining the traces. It is realized, of course, that the shutters could be actuated to entirely eliminate the zero lines or traces, but the presence of these traces prevents film breathing.

Thus, fromthe above description it will be observed that a print is provided in which, at times of no signahgroundnoise has been'reduced to a minimum, while the peaks of the modulations have not been unnecessarily clipped by the attempt of the shutters to move out of the way at thebeginning of the signal from their normal nosignal position.- In other words, the track area is prepared 'forthe impression of the signal and no clipping will occur except at the rare intervals such as gun-shots or otherbackground noises wherein distortion is not detrimental when these sounds are'reproduced. Another advantage of this type of noise reduction action is that it is unnecessary to increase the opening period or the steepness of the noise reduction action between the points b-b and 11-41, which would introduce audible plops or clicks. The slow closing at the end of the signal also prevents cancellation oflow frequency fundamentals,which, of course, is also undesirable.

Referring now to theother dotted portion of Fig. 1, a source of potential 48, a resistance 49 and a manual switch 50, are connected in series to a coil of the galvanometer l2, this coil affecting the deflection of the galvanometer II. By the use of the circuit 48, 49 .and 50 the same general result may be accomplished as was accomplished by the three control circuits above-described. That is, either circuit 48, 49 and 50 or one of the first three control circuits may be employed.

' The resulting sound record of the system under control of circuit 48, 49 and 50 is shown in Fig 3, the negative having been run upwardly through the recorder, as shown by thearrow. In this case, however, the opaque areas produced by the shutters l4 are of the normal type having a faster opening time than closing time, but of slightly narrower width. It will be noted that from the points a-a' upwardly the same width transparent traces as shown in Fig. 2 are present, these traces, however, being nearer the edges of the sound track area. The location. of these traces is controlled by the bias applied to the tube 33 and the positioning of the mirror of galvanometer l2 with respect to the slit l8, as is well known in the art.

To prepare the sound track for the signal in this instance, however, the key 50 is opened or closed, depending on the polarity of source 48, which allows the galvanometer to be tilted so as to provide traces of a width shown between aa' and b'b', which are similar in width to the traces in Fig. 2 between the same unprimed numbers. With the increased width, the modulations of the signal may now be impressed with the minimum of interference from the shutters, as in Fig. 2. The shutters begin separating, upon impression of the signal, as shown at points b-b' and continue to open and close for the different signal levels at the rate set by filter timing elements 29, 30 and 3|. In the print shown, the shutters reach their normal no signal position at points e'-e, and when switch 50 is closed at some later time, such as at points f'f, the original narrow zero traces are again produced. Thus, a similar overall type of track is provided in Fig. 3 as in'Fig. 2, the operation of preparing the track area for the incoming signal being performed by the galvanometer 12 instead of by the noise reduction shutters l4, both records, however, having the sameadvantages as mentioned above.

Referring now to Figs. 4, 5 and 6, a recording system is shown wherein the advantages of the records of films shown in Figs. 2 and- 3 are obtained automatically. In Fig. 4 the right-hand portion of the recording system corresponds to' the right-hand portion of Fig. 1, like elements being given the same numerals. The recording circuit comprises a microphone 55 and amplifier 56, the main output of which is fed over conductors 51 to the coil 58 of the galvanometer. The

remaining portion of the output of amplifier 56 tential source 61 over adjustable potentiometer 68,this potentiometer controlling the initial seppoints a aration of shutters 14 in the same manner as in Fig. 1. The channel has been indicated as; channel A for later discussion. I

The other portion of the output of amplifiier 563 is fed into channel B, consisting of an amplifier 10 connected through transformer 69 to a rec tifier H similar to rectifier'lill, andto a direct" current amplifier 12 similar to amplifier'fil. In termediate the tubes H and I2 is a filter timing circuit composed of resistancesl3 and I4 and a condenser 15 similar to the elements 63, 64 and 65 of channel A. A biasing potential is impressed on the grid of tube 12 from a potential'source Tl shunted by resistance 18 to also control shutters 14. Thus, there are two noise reduction units or channels, A and B, connected in parallel between the output of amplifier 56 and the winding [5 of the noise reduction shutters l4. Attention is directed to the setting of the biasing poten tiometers and it. will be noticed that the adjust able contact on resistance 68 is near the posi-- tive terminal of battery .61, while the adjustable: contact 8| on the resistance 18 is near the nega-- tive terminal of battery 11. The different bias settings for the tubes BI and 12 of channels A and, B, plus the use of amplifier 10 in channel B, produce a variation of current through the coil l5 and actuation of the noise reduction shutters I4,as will now be described by reference to Figs. 5 and 6.

In Fig. 5 a sound record made on the system of Fig. 4 is shown with opaque portions 84 and 85 and a central opaque portion 86. Two transparent traces similar to the ones in Figs. 2 and 3 are produced at times of no signal, the film having been recorded in the direction of the arrow. At a" the signal is impressed on the galvanometer 58 and, substantially simultaneously,'the rectified currentsfrom the tubes 6t and 12 are impressed on the shutter winding [5. However, due to the amplifier 10 or unit B, the setting of the slider 8| and the Value of the elements "I3, 14 and 15, the shutters are actuated very rapidly with respect to the increase'in amplitude of the signal 0 so that the first modulations of the signal are very effectively accommodated without clipping. This rapid action is not, however, permitted to continue throughout the track area since it would introduce deleterious audible frequencies because of its form. This limiting control is set by the. bias on tube 12, so at points bb", where the signal has increased to a predetermined level, the noisereduction channel A takes over control and the shutters [4 are separated solely from the output of this channel and follow the usual rate of opening, which has been found suitable without introducing extraneous noise. This opening rate is shown between the points b"b*' and dd, while the closing action is illustrated beyond the points d--d" as being at the usual slower rate for the first portion and then more rapid as channel B assumes control. I

To further illustrate the action of the parallel units, reference is made to Fig. 6, which is a graph between the input voltage and galvanometer and shutter deflections. Curve S represents the movement of the galvanometer with respect to the signal input voltage and is shown as linear between zero input voltage and full-track or modulation. Starting from zero input, the curve B illustrates the rapid shutter deflection in comparison with the deflection of the galvanometer caused by the increase in amplitude of the input signal. This shutter deflection. is primarily caused by the output of channel B of Fig. 4. Curve A is the result of the action of channel A and although its slope is comparable to the slope of the galvanometer deflection, the shutter will lead the galvanometer deflection since the shutter was given an initial higher acceleration. To obtain the optimum setting points of the respective channels, We might refer to point 9', where the input voltage corresponds to 100% modulation of the track area. It will be noted that the deflection of shutters M leads the deflection of the galvanometer caused by the signal amplitude by 4 db. for full-track modulation. This is the clearance provided for safe operation and to insure that the shutters have moved out of the way before the signal reaches 100% modulation. Now, it has been found that the optimum point for channel A to take over is at substantially 30 db. below full-track modulation, as shown at point k. These settings have resulted in particularly satisfactory operation.

Referring now to Figs. 7, 8 and 9, illustrating a third recording system for producing the optimum in noise reduction action automatically, the right-hand portion of Fig. 7 is identical to the right-hand portions of the systems shown in Figs. 1 and 4. In this embodiment, the output of microphone 90 is amplified by amplifier SI, and a portion thereof fed to coil 92 of the galvanometer over conductors 93. The remaining portion of the output of amplifier 9! is impressed on a rectifier tube 95 through a transformer 96 and then fed to a pair of direct current amplifier tubes 91 and 93 connected in parallel. Although these tubes are shown as of the screen grid variable mu type, the three electrode type of tube may also be employed. Furthermore, one or more tubes connected in parallel may be used. Intermediate the rectifier 95 and amplifiers 9'! and 98 is the usual timing filter circuit composed of resistances I and [0| and condenser 32. Bias for the amplifiers 9! and 98 is obtained from abattery N15 or other suitable potential source shunted by a potentiometer Hit. The output current for operating shutter I is obtained directly from the plates of tubes 97 and 98 and from the screen grids through a resistance Hi8.v

The screen grids and plates may also be directly connected together. The coil [5 is shown connected to the negative terminal of the plate and screen grid potential source for the tubes 91 and 98.

The primary difference between this recording system and the above-described systems is in the noise reduction portion and particularly in the tubes 9! and 8. Instead of these tubes being of the type in which the characteristic between input voltage and output current has a linear portion, these tubes are of the exponential or variable mu type having a drooping curved characteristic. Tubes of this exponential type and their characteristic are disclosed in my Patent 2,006,052, of January 25, 1925, and are commercially known as type 6K7.

Thus, because of the particular relationship between the input and output of these amplifiers, the current feed to the coil l5 of the noise reduction shutters I4 is not proportional to the input. to the rectifier 95 and, consequently, the deflection of the shutters with respect to the defiection of the galvanometer varies in a non-linear manner. The particular form of this variation is shown in Fig. 9, which is a graph similar to the graph of Fig. 6 wherein curve C represents the galvanometer deflection with respect to input voltage, while curve D represents the shutter deflection with respect to input voltage. It is to be noted that the rate of movement of the shutter for equal signal input voltages over thelower portion of the amplitude range is different from the rate of shutter movement above this range. In other words, the shutter action is more rapid at the lower amplitudes, so. that the shutter quickly moves out of the way to clear the track area for the modulations. After a certain clearance or lead has been obtained,.the rate of shutter action becomes slower and may become proportional and less than the rate of galvanometer defiection. In Fig. 9 the shutter is shown clear of the track area at a point approximately 3 db. below 100% modulation, this amount of clearance being satisfactory because of the non-linear relationship between the input voltage and the shutter deflection. Thus, this noise reduction action provides a rapid opening over a portion of the track area without introducing noise because the shutter speed is reduced over the remaining.

portion of the track area.

For the type of record print obtained with the system of Fig. 7, reference is made to Fig. 8 wherein it will be noted that although the shutter action begins substantially simultaneously with the modulations, the first portion of the movement is at a much greater rate than the latter portion of the deflection. This record also shows that the timing elements I00, I0] and I02 provide a slower overall closing than opening time for the shutters.

This system, therefore, provides a third arrangement for obtaining the optimum action in the application of noise reduction. Although the above three modifications produce the results desired in different forms, the preferred modification will be somewhat dependent upon operating convenience and equipment available. In each case, however, both peak-clipping and ground noise are reduced to a practical minimum without the introduction of undesired noise. This has been accomplished in each case by either preparing the track area for the light modulations just prior to the impression of the signal on the galvanometer or operating the shutters at a rapid rate during the lower amplitudes of the signal and then reducing the speed of shutter movement as the signal amplitudes further increase.

It is also to be understood that the above noise reduction actions are applicable to variable density recording in the same manner as described above and with like advantages, the modulation point being varied in the variable density systems instead of the shutters.

I claim:

1. In a sound recording system, the combination of means for generating electrical currents, means for producing a light beam, means for varying said light beam in accordance with the instantaneous values of said currents, means for varying said light beam in accordance with the average value of said currents, both of said light beam varying means being actuated by said respective currents substantially simultaneously, and means for manually controlling said lastmentioned means whereby the action of said lastmentioned means anticipates the action of said first-mentioned light beam varying means.

2. A sound recording system in accordance with claim 1 in which said last-mentioned means include a shutter in said light beam, said shutter being under control of said manual control means tea predetermined extent and under control-of the average valueof saidcurrents' to a second predeterminedextentfl r g 3. In a sound recording system, the combination of means for generating electrical currents, means for producing a light beam, means 'for varying said light beam in accordance with the instantaneous values of said currents, means for varying said light beam in accordance with the average value of said currents, both of said light beam varying means being actuated substantially simultaneously by said respective currents, and means included in said last-mentioned means whereby said last-mentioned means is controlled at one rate over a range of operation lying substantially below 30 db. under 100% modulation of said system and at a different rate over a range of operation lying between substantially 30 db. under 100% modulation and substantially 4 db. under 100% modulation of said system.

4. A sound recording system in accordance with claim 3 in which said included means comprises independent parallel electrical channels having different characteristics between the respective inputs and outputs thereof, one of said channels being effective at low amplitudes and the other at the higher amplitudes of said currents.

5. The method of sound recording comprising generating electrical currents corresponding to the instantaneous values of sound Waves to be recorded, varying a light beam in accordance with the instantaneous values of said currents, carrying said light beam in accordance with the average value of said currents, and increasing the differential change between the instantaneous varying of said light beam and the average varying of said light beam, the increasing of said differential being at a faster rate over a low range of amplitudes of said currents than over a higher range of amplitudes of said currents.

6. A film sound record having a sound track thereon in which the average transparency has a low constant value during times of no signal, a high constant value during times of said signal, and an intermediate constant value just prior to and after said signal.

7. The method of sound recording comprising generating electrical currents corresponding to the instantaneous values of sound waves to be recorded, varying a light beam in accordance with said currents, varying said light beam in accordance with the average value of said currents during the instantaneous varying of said beam by said currents, and manually controlling the varying of said light beam before and after said beam is varied by said currents.

8. A film sound record having a sound track thereon in which the average transparency has a certain constant value during times of no signal, a different substantially constant value during said signal, and a still different constant value just prior to and after said signal.

9. A sound recording system comprising means for generating electrical currents, means for producing a light beam, means for projecting said light beam on a light-sensitive material, means for varying said light beam in accordance with the instantaneous values of said currents, means for varying said light beam'in accordance with the average value of said currents, a rectifier for said originally generated currents, means for connecting the output of said rectifier to said second-mentioned light beam varying means and manually operated means for energizing said second-mentioned light beam varying meansindependently-of the rectified currents'irom said rectifier. v k I lQqA sound recording system inaccordance with claim 9 in which said manually operated means-comprisesa circuit including apotential means for projecting a light beam from said light source upona light-sensitive material, means for varying said light beam by said electrical currents in accordance with the instantaneous values thereof, a second means independent. of said first-mentioned light-varying means for varying said light beam, said second means being actuatedby the average value of said currents, and means interposed between said second light beam varying means and said source of electrical currents, said last-mentioned means comprising a plurality of independent electrical channels having diiierent characteristics between respective inputs and outputs thereof, the outputs being connected to said second light beam varying means, and the inputs to said source of electrical currents.

12. A sound recording system in accordance with claim 11 in which each of said channels is connected in parallel and includes an independent rectifier for said signaling currents, and independent timing means for controlling the rate of impression of the respective rectified currents on said second means.

13. A sound-recording system comprising a source of electrical currents, a lightsource, means for projecting a light beam from said light source to a light-sensitive material, means for modulating said light beam in accordance with the instantaneous values of said electrical currents, said instantaneous values being substantially directly proportional and substantially linearly related tothe average variations in amplitude of said currents, a second light beam modulating means, and means interposed between said second light beam modulating means and Said source of electrical currents, said means comprising a rectifier and means for amplifying said rectified currents, said amplifying means producing a nonlinear and non-proportional relationship between the average value of said electrical currents and modulation of said light beam by said second light beam modulating means. f

14. A sound recording system in accordance with claim 13 in which said amplifying means for producing a non-linear and non-proportional relationship between the average value of said electrical currents and the modulation of said light beam by said second light beam ,modulating means comprises at least one exponential vacuum tube connected intermediate said rectifier and said second light beam modulating means.

15. In a sound recording system, the combination of means for translating sound waves into electrical currents, means for producing a light beam, means for varying said light beam in accordance with the instantaneous values of said currents, means for varying said light beam in accordance with the average value of said currents, both of said light beam varying means being actuated by said respective currents substantially simultaneously, and means for extraneously and independently actuating said average current of said light beam varying means by said respeetivecurren 16. The method of sound recording comprising translating sound waves into electrical currents, varying a light beam in accordance with the instantaneous values of said currents, varying said light beam in accordance with the average value of said currents during the instantaneous varying of said beam by said currents, and extraneously independently varying said light beam a predetermined amount immedately prior to the variation of said beam by said respective light beam varying currents.

BARTON KREUZER. 

